Short-circuit releases are employed in power circuit-breakers for switching and protecting motors and other loads. The short-circuit releases are designed as electromagnetic tripping devices substantially comprising a coil winding, a coil former, an armature, a pole, a plunger, a restraining spring, and a yoke. The armature is attracted at a specific rated current of the power circuit-breaker, for example at twelve times the nominal current in the case of motor protection or nineteen times the nominal current in the case of transformer protection. The armature's motion therein acts upon a breaker mechanism and a moveable contact member in order to open the contacts. The relevant standard therein specifies that the operating current can vary by at most +/−20%.
Because of the requisite larger supporting cross-sections, smaller numbers of turns, wider tolerances for the coil and wrap wire, and the greater magnetic-field inhomogeneity associated therewith, the difficulty with relatively wide adjustment ranges lies in positioning the coil winding with respect to the air gap between the armature and pole sufficiently accurately to enable the operating limits to conform to the relevant standard. There is also the problem of fixing the coil winding in its determined position with respect to the air gap so that the coil winding will not become displaced in the direction of the center of gravity of the iron at the rated current or when the short-circuit current is high, or if the coil contracts and becomes deformed with the result that the operating limits will then no longer be adhered to.
For higher breaking capacities the coils are produced having the winding turns resting on them so that the coil cannot contract and become deformed in the event of high short-circuit currents. Owing to the use of uniform coil formers for the respective structural size and the fact that they are designed for the geometrically largest coil winding, there is often a gap between the coil-former flange or yoke and the last turn of the coil winding. In order to fix the coil winding once it has been positioned accurately with respect to the air gap between the armature and pole, one of the coil winding's ends is bonded to the coil-former flange or yoke and the other is welded to a terminal.
The short-circuit release has to be matched in the case of power circuit-breakers having a high breaking capacity, for example up to 100 kA for a nominal current of 80 A. The high breaking capacity of an 80-A device makes its heat management critical. Moreover, the force requirements on the release are also increased because of the growing contact load. The magnetic circuit needs to be more efficient in its structural design given that the release's power dissipation must not exceed that of today's 50-A release with a field strength correspondingly the same.
Present-day switching devices have a relatively poor magnetic circuit because the yoke plate is at the same time embodied as a bimetallic support and made from a platinized material, for example an iron/copper mating.
That problem of a relatively poor magnetic circuit has hitherto been resolved by way of a platinized copper/steel plate and a corresponding arrangement of the parts. Partially bonded materials can furthermore be produced and then shaped appropriately arranged for the required material mating. Likewise used are differently shaped yoke plates which encompass the release's core consisting of an armature and pole in order to interact with the coil and achieve a release effect. The force level of said releases is maintained by a field strength that is highly dimensioned. That means an inefficient energy conversion between electrical and mechanical energy.